1 / 24

A Comparison of Software and Hardware Techniques for x86 Virtualization

A Comparison of Software and Hardware Techniques for x86 Virtualization. By Keith Adams and Ole Ageson VMWare. Presented by Mike Marty. The Renaissance of Virtualization. 1970s: virtual machines first used 1990s: x86 becomes prominent server platform No vertical integration in x86

nate
Download Presentation

A Comparison of Software and Hardware Techniques for x86 Virtualization

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. A Comparison of Software and Hardware Techniques for x86 Virtualization By Keith Adams and Ole Ageson VMWare Presented by Mike Marty

  2. The Renaissance of Virtualization • 1970s: virtual machines first used • 1990s: • x86 becomes prominent server platform • No vertical integration in x86 • Lack of enterprise features in commodity OSs • 1999: VMWare first product to virtualize x86 • 2006: AMD and Intel offer hardware support

  3. Outline • Classic Virtualization • Software Virtualization • Intel/AMD Hardware Virtualization • Comparison and Results • Discussion

  4. Classic Virtualization • Popek and Goldberg’s Criteria: • Fidelity – run any software • Performance – run it fairly fast • Safety – VMM manages all hardware • Trap-and-Emulate only real solution until recently

  5. Trap-and-Emulate Virtualization 1. De-Privilege OS user mode apps OS kernel mode

  6. Trap-and-Emulate Virtualization 1. De-Privilege OS apps apps user mode OS OS virtual machine monitor kernel mode

  7. Trap-and-Emulate Virtualization 1. De-Privilege OS 2. Shadow structures and memory tracing shadow page table shadow page table apps apps user mode OS OS primary page table virtual machine monitor kernel mode

  8. Trap-and-Emulate cont. • Traps are expensive (~3000 cycles) • Many traps unavoidable • E.g., page faults • Important enhancements • “Paravirtualization” to reduce traps (e.g., Xen) • Hardware VM modes (e.g., IBM s370)

  9. Can x86 Trap and Emulate? • No • Even with 4 execution modes! • Key problem: dual-purpose instructions don’t trap • Classic Example: popf instruction • Same instruction behaves differently depending on execution mode • User Mode: changes ALU flags • Kernel Mode: changes ALU and system flags • Does not generate a trap in user mode

  10. Outline • Classic Virtualization • Software Virtualization • Intel/AMD Hardware Virtualization • Comparison and Results • Discussion

  11. Software Virtualization with VMWare • Binary translation! (mostly safe, user-mode) X86 X86

  12. VMWare’s Binary Translation • On-the-fly • Only need to translate OS code • Makes SPEC run fast by default • Most instruction sequences don’t change • Instructions that do change: • Indirect control flow: call/ret, jmp • PC-relative addressing • Privileged instructions • Adaptive Translation • “Innocent until proven guilty”

  13. Performance Advantages of BT • Translation sequences can be faster than native: • cli vs. vpu.flags.IF := 0 • Avoid privilege instruction traps • Example: rdtsc • Trap-and-emulate: 2030 cycles • Callout-and-emulate: 1254 cycles • BT emulation: 216 cycles (but TSC value is stale)

  14. Outline • Classic Virtualization • Software Virtualization • Intel/AMD Hardware Virtualization • Comparison and Results • Discussion

  15. AMD SVM and Intel VT • Extensions to x86-32 and x86-64 • Allows classic trap-and-emulate! • Hardware VM modes to reduce traps • Details: • VMCB – virtual machine control block • VMX mode for running guest OSs • Vmrun instruction to enter VMX mode • Many instructions and events cause VMX exits • Control fields in VMCB can change VMX exit behavior

  16. Hardware VM Example: syscall • VMM fills in VMCB exception table for Guest OS • Sets bit in VMCB notexit on syscall exception • VMM executes vmrun • Application invokes syscall • CPU  CPL #0, does not trap, vectors to VMCB exception table

  17. Software BT vs. Hardware VM • Binary Translation VMM: • Converts traps to callouts • Callouts faster than trapping • Faster emulation routine • VMM does not need to reconstruct state • Avoids callouts entirely • Hardware VMM: • Preserves code density • No precise exception overhead • Faster system calls

  18. Compute-bound Benchmarks Bottomline: little difference for SPEC

  19. Mixed Benchmarks Cygwin Make is SLOW! Process-based Thread-based Who Cares? Would Hardware VM do better for multithreaded database?

  20. Costs of Operations

  21. Nanobenchmarks

  22. VMWare Nanobenchmarks • syscall • Native/Hardware VMM: same • Software VMM: +2000 cycles • in • Native: 3209 cycles • Hardware VMM: 15826 cycles • Software VMM: 15x faster? • call/ret • Native/Hardware VMM: 11 cycles • Software VMM: 51 cycles

  23. Opportunities • Faster Microarchitecture implementations • Intel Core Duo already much faster than P4 • Hardware VMM algorithms • Software/Hardware Hybrid VMM • Hardware MMU • Virtualize DMA

  24. Catalysts for Discussion • Is BT really faster for things that matter? • Process-based Apache on Linux? • Who configures a system to constantly page? • VMWare is done, why bother with Hardware VM support? • Simplicity of VMM w/ Hardware support • New applications • Will next-gen hardware make binary translation unnecessary?

More Related